A. Field
This invention generally relates to methods and apparatuses for electrical wire, and more particularly to insulated wire for fire safety cable.
B. Description of the Related Art
Fire safety cable [critical circuit-integrity cable)] finds application in providing electrical power to equipment and systems that are required to function during a fire. These systems may include fire alarm controllers, fire suppression equipment, sprinkler pumps in high rise buildings or other environments. This equipment needs to operate for a sufficient period of time to allow the safe evacuation of people the location of the fire.
Fire performance cables are required to continue to operate and provide circuit integrity when they are subjected to fire. To meet some of the standards, cables must typically maintain electrical circuit integrity when heated to a specified temperature (e.g. 650, 750, 950, 1050° C.) in a prescribed way for a specified time (e.g. 15 minutes, 30 minutes, 60 minutes, 2 hours). In some cases the cables are subjected to regular mechanical shocks, before, during and after the heating stage. Often they are also subjected to water jet or spray, either in the latter stages of the heating cycle or after the heating stage in order to gage their performance against other factors likely to be experienced during a fire.
These requirements for fire performance cables have been met previously by wrapping the conductor of the cable with tape made with glass fibers and treated with mica. Such tapes are wrapped around the conductor during production and then at least one insulative layer is subsequently applied. Upon being exposed to increasing temperatures, the outer insulative layers are degraded and fall away, but the glass fibers hold the mica in place. These tapes have been found to be effective for maintaining circuit integrity in fires, but because of the additional manufacturing steps they are quite expensive to produce. Further the process of wrapping the tape around the cable is relatively slow compared to other cable production steps and thus, wrapping the tape slows overall production of the cable further adding to the costs. Attempts have been made to reduce the costs by avoiding the use of tape and extruding a cable coating consisting of a flexible polymeric composition which forms an insulating ceramic when exposed to fire to provide the continuing circuit integrity.
In the past the electrical power was provided through the use of mineral insulated cable. This cable required a very complex installation and retrofitting the existing structure was difficult and cost prohibitive. More recently, new and improved wire insulation material has been introduced for the safety cable (critical circuit) application. Today the material of choice for wire insulation is a silicone rubber that has been specially formulated to form a ceramic-like layer when heated to the temperatures that are present in a fire.
The wire construction for safety cable (CI—“circuit integrity”) is typically a copper conductor. Over the copper conductor is applied the ceramifiable silicon rubber insulation. A jacket material is applied over the silicone insulation to provide mechanical protection during installation. While the ceramifiable insulation performs adequately in a fire, this material is marginal for other electrical requirements for the cable. One safety cable (CI) requirement for this family of cables is a fire test where the cables are installed in a manufacturer's specified system, and then tested for functionality in a furnace that models petroleum-fueled fire. In one test protocol the furnace is programmed to subject the test samples to a temperature rise on ambient to 1010° C. over a period of 2 hours. During this test the cables are energized to the voltage appropriate to the cables specified application. One test used is UL 2196 for 2 hours. To meet the requirements of the UL2196 test, electrical functionality must be maintained throughout the 2 hours and the following simulated fire hose water spray test.
The UL2196 test method described in these requirements is intended to evaluate the fire resistive performance of electrical cables as measured by functionality during a period of fire exposure, and following exposure to a hose stream. To maintain the functionality of electrical cables during a fire exposure the cables are tested using a fire resistive barrier. The fire resistive barrier is the cable jacketing if the jacketing is designed to provide fire resistance. If the cable jacketing is not designed to provide fire resistance, the electrical cables are either placed within a fire resistive barrier or installed within an hourly rated fire resistive assembly. Fire resistive cables intended to be installed with a non-fire resistive barrier (such as conduit) are tested with the non-fire resistive barrier included as part of the test specimen. Otherwise fire resistive cables incorporating a fire resistive jacket are tested without any barrier. To demonstrate each cable's ability to function during the test, voltage and current are applied to the cable during the fire exposure portion of the test, and the electrical and visual performance of the cable is monitored. The cable is not energized during the hose spray, but it is visually inspected and electrically tested after the hose spray. The functionality during a fire exposure of non-fire resistive electrical cables which are intended for installation within fire barriers or for installation within hourly rated fire resistive assemblies is determined by tests conducted in accordance with the UL Outline of Investigation for Fire Tests for Electrical Circuit Protective Systems, Subject 1724. Two fire exposures are defined: a normal temperature rise fire and a rapid temperature rise fire. The normal temperature rise fire is intended to represent a fully developed interior building fire. The rapid temperature rise fire is intended to represent a hydrocarbon pool fire. Two hose stream exposures are defined: a normal impact hose stream and a low impact hose stream. The low impact hose stream is applied only to cable intended to contain the identifying suffix “CI” to identify it as CI cable in accordance with the Standard for Cables for Power-Limited Fire-Alarm Circuits, UL 1424, and in accordance with the Standard for Cables for Non-Power-Limited Fire-Alarm Circuits, UL 1425. In addition to fire alarm cables referenced in UL 1424 and UL1425, power cables can also be approved for CI (critical circuit) applications. These power cables must meet the performance requirements listed in UL 44. Type RHH, RHW2, RHW and others of this standard if able to pass UL2196 can be qualified for CI applications.
In addition to the UL 2196 test, the safety cable (CI) must also meet the electrical requirements for non-CI rated cable. One of the requirements for this family of cables is long term insulation resistance. For this test, a copper conductor, with only the silicone rubber used as insulation, is tested at the specified voltage while the cable is immersed in 90° C. water. The insulation resistance is monitored periodically. The decrease in resistance must level out at a value above the minimum required. One of the requirements is specified in UL 44. The ceramifiable silicone rubber used in this application is a compound supplied by Wacker. They are a major supplier for insulation compounds for safety cable (CI). This compound can pass the requirements of UL 2196, but is marginal to unable to meet the requirements of UL 44 for insulation resistance long term in 90° C. water at rated voltage.
This UL44 test specifies the requirements for single-conductor and multiple-conductor thermoset-insulated wires and cables rated 600 V, 1000 V, 2000 V, and 5000 V, for use in accordance with the rules of the Canadian Electrical Code (CEC), Part 1, CSA C22.1, in Canada, Standard for Electrical Installations, NOM-001-SEDE, in Mexico, and the National Electrical Code (NEC), NFPA-70, in the United States of America.
Uncured silicone rubber contains polymers of different chain lengths. It comprises a principal silicon-oxygen chain (the siloxane backbone) and an organic moiety bound to the silicon. A silicon atom has four valence electrons, which is why silicon rubber is often abbreviated with a Q for “quaternary group.” The properties of silicone rubber vary greatly depending on the organic groups and the chemical structure. The organic groups may be methyl, vinyl, phenyl, or other groups. Depending on which organic groups are present, silicone polymers in common use are classified as follows: MQ, or polydimethylsiloxane (PDMS), denotes a polymer in which two methyl groups are bound to the siloxane backbone; VMQ stands for polydimethylsiloxane in which a small number of methyl groups have been replaced by vinyl groups; PVMQ stands for a VMQ in which a small number of methyl groups have been replaced by phenyl groups; and FVMQ stands for a VMQ in which a small number of methyl groups have been replaced by trifluoropropyl substituents.

Ceramifying polymer materials have been developed by incorporating ceramic forming pre-cursors into thermoplastics. These compounds can be processed on conventional plastic extrusion equipment to form sheets, profiles or coatings. In a fire situation, the polymer component is quickly pyrolized. However, a porous, coherent ceramic begins to form at sufficiently low temperatures to maintain the structural integrity of the material through to temperatures of over 1000° C. The ceramic forming systems can be adjusted to minimize dimensional changes, or to provide a degree of intumescence through entrapment of volatile gases from the polymer. This can produce a cellular structure with increased thermal resistance. Ceramifying polymer technology has already been commercialized for fire resistant cable coatings and shows promise for many other fire protection coating applications.
Ceramifying polymers generally consist of a polymer matrix with refractory silicate minerals which form the ceramic framework in combination with a flux system. This can allow a coherent ceramic structure to form at a relatively low temperature. Other functional additives may be added including stabilizers and flame retardants. Although the total ceramifying additive level must be quite high, the materials can still be processed like conventional plastics. A wide range of ceramifying polymers can be produced, including thermoplastics and emulsions suitable for coatings. Ceramification can be combined with intumescence through a mechanism which traps volatiles from the polymer decomposition as the ceramic structure is formed. This can produce a strong, cellular coating layer with good thermal resistance for fire protection applications.
Ceramifying polymers are not inherently flame retardant. However, they can be modified with organic or inorganic flame retardant systems to achieve low flammability ratings. Ceramification can also assist fire performance by producing a stable surface layer which insulates the underlying layers and may inhibit volatile emissions. This can delay ignition and reduce heat release rates.
Most polymers begin to decompose through oxidative reactions at temperatures of around 200° C. Higher performance polymers such as silicones persist to over 300° C. But typical fire tests require exposure to a temperature profile based on the combustion of a cellulose fuel load in a representative room. This reaches 700° C. in about 10 minutes at which all polymers, including silicones, rapidly decompose. The temperature continues to increase to 1000° C. after 1 hour. Hence, conventional polymers are generally unable to provide a barrier to fire, or thermal insulation, in systems which require a rating of 60 minutes or longer in these tests. These fire ratings are usually achieved by using intumescent materials, which produce an inorganic char with limited cohesive strength, or thick protective structures made from gypsum board or similar materials.
A characteristic of ceramifying polymers is their ability to form a self-supporting structure throughout the temperature range from ambient service temperature to over 1000° C. Reactions in the inorganic ceramic forming systems can commence from temperatures as low as 350° C. and continue to 800° C. or higher. This is achieved with fluxes which produce a controlled, low level of liquid phase at these temperatures. Ceramification in these materials is not simply the bonding or fusing of the silicate particles by a viscous liquid phase, such as with relatively high levels of low melting point glasses. Such materials tend to collapse at high temperatures and are not self-supporting. Ceramification involves reaction sintering assisted by the controlled level of liquid phase.
Solid silicone rubber contains polymers with a high molecular weight and relatively long polymer chains. Silicones are characterized by a fully saturated backbone of alternating silicon and oxygen atoms. The Si—O links in the chain have an exceptionally high bond energy of 451 kJ/mol. C—C links, by comparison, have a bond energy of 352 kJ/mol. The Si—O chain mobility is very high. The organic side groups shield the backbone, so surface energy is low.
Crosslinkers are typically utilized to convert the raw rubber into a mechanically stable cured product. Use is made of peroxide or platinum catalyst systems. Fillers are also used to reinforce the elastic silicone network. Peroxide curing involves the use of organic peroxides. At elevated temperatures, they decompose to form highly reactive radicals which chemically crosslink the polymer chains. The result is a highly elastic, three-dimensional network. During platinum-catalyzed addition curing, the crosslinker's Si—H groups react with the vinyl groups of the polymer to form a three-dimensional network.
